The present invention is directed to control devices for bicycle transmissions and, more particularly, to an apparatus that compensates for return spring and other forces applied by the bicycle transmission to a motor drive used to operate the bicycle transmission.
There are two major types of bicycle transmissions currently marketed. One type of bicycle transmission uses a derailleur to shift a chain from one sprocket to another to change the drive ratio, and the other type uses a planetary gear mechanism disposed inside a rotating hub such as a wheel hub to change the drive ratio. In either case, an operating member such as a lever, push rod or rotatable ring is moved to operate the transmission, and the operating member is connected to a transmission actuating member such as a control wire or link that is pulled or released by the rider to cause the desired movement of the operating member. To facilitate the operation of the transmission in a pull and release manner, a return spring commonly is used to bias the operating member to an initial state. The transmission then is operated by moving the operating member against the progressively increasing force of the return spring and the natural resistance by the transmission to changing gears. When the resistance of the return spring is added to the natural resistance by the transmission to changing gears, the force that must be applied to the transmission actuating member can become quite large.
Motor-driven bicycle transmissions are becoming more common. Such motor-driven bicycle transmissions relieve the rider of the task of manually pulling and releasing the transmission actuating member. Of course, the motor used to cause the pulling and releasing of the transmission actuating member must be designed to withstand the forces noted above. This sometimes makes it necessary to use a relatively large motor which increases the cost, size and weight of the transmission operating mechanism. The larger motor also creates significantly more current drain on the battery used to operate the motor, thus resulting in short battery life. Consequently, motor-driven transmissions usually are not used when the size, cost or weight of the bicycle is an important factor, such as in racing environments.